1. Technical Field
The present invention relates to an approach method for a probe and a sample in a scanning probe microscope for relatively scanning the probe and the sample to measure a shape and a physical property of a surface of the sample, process the surface of the sample, or move a substance on the surface of the sample by the probe while controlling a distance by detecting an interaction between a cantilever including the probe at its distal end and the surface of the sample in the atmosphere, a vacuum, or a solution.
2. Description of the Related Art
Referring to FIGS. 5 and 6, a configuration of a conventional scanning probe microscope is described (see JP 2007-33321 A).
FIG. 5 is a configuration diagram of the conventional scanning probe microscope. In the conventional scanning probe microscope, a sample 101 is placed on a distal end of a three-axis fine-movement mechanism 102 including a cylindrical piezoelectric element, the three-axis fine-movement mechanism being fixed on a coarse-movement mechanism 103 used for allowing the sample 101 and a probe 106b provided at a distal end of a cantilever 106a to be described below to approach each other. On the other hand, the cantilever 106a including the probe 106b at its distal end is placed immediately above the sample 101, the cantilever 106a including a piezoelectric element 105 for exciting the cantilever at its proximal portion. A displacement of the cantilever 106a is detected by a displacement detecting mechanism including a laser diode (LD) 104 and a photodetector (PD) 107. The displacement detecting mechanism generally employs a method called optical lever method to reflect a laser beam from the LD 104 on a back surface of the cantilever 106a and cause the reflected beam to enter a detection surface of the PD 107. When the cantilever is deflected, a laser spot on the detection surface of the PD 107 moves. The displacement of the cantilever 106a may be detected by the spot position on the detection surface at this time.
A case where an apparatus having the above-mentioned configuration is used for measurement by an oscillating mode atomic force microscope, which is one type of the scanning probe microscope, is described. An amplitude and a phase of the cantilever are measured by the displacement detecting mechanism while exciting the cantilever 106a in the vicinity of a first-order resonant frequency by the piezoelectric element 105, the sample 101 is allowed to approach the probe 106b by the coarse-movement mechanism 103, and thereafter the probe 106b and the sample 101 are allowed to further approach each other by the three-axis fine-movement mechanism 102. Then, a physical force such as an atomic force acts between the sample and the probe 106b. When the sample and the probe 106b further approach each other, the sample and the probe 106b intermittently contact each other in accordance with the oscillation of the cantilever 106a and a contact force acts between the sample and the probe 106b. The atomic force and the contact force change the amplitude and the phase or the resonant frequency of the cantilever 106a. The amount of change depends on the distance between the probe 106b and the sample 101. Therefore, in order to ensure that the amount of change in amplitude and phase or resonant frequency of the cantilever 106a is always constant, the distance between the probe 106b and the sample 101 is controlled by the three-axis fine-movement mechanism 102 to control the distance in a height direction. Further, a topographic image of a sample surface may be measured by scanning the probe 106b within a plane of the sample 101 by the three-axis fine-movement mechanism 102.
In addition to the oscillating mode atomic force microscope, there is also known a method called contact mode. In this method, the probe and the sample are brought closer to each other by the coarse-movement mechanism while detecting the displacement by the displacement detecting mechanism without exciting the cantilever, and thereafter the distance in the height direction is controlled by the three-axis fine-movement mechanism. At this time, the physical force such as the atomic force acts on a tip of the probe, and the probe experiences an attractive force at first and experiences a repulsive force when the probe and the sample further approach each other. The attractive force and the repulsive force deflect the cantilever. The physical force such as the atomic force depends on the distance between the probe and the sample. Therefore, the probe and the sample are allowed to approach each other within an area in which the atomic force acts on the probe, the probe is scanned within a two-dimensional plane by the three-axis fine-movement mechanism, and the distance between the probe and the sample is controlled so that the amount of deflection of the cantilever becomes constant, to thereby form the topographic image of the sample surface.
In the case of the oscillating mode atomic force microscope, there is also known a method of performing a measurement while exciting the cantilever in the vicinity of a second- or higher-order resonant frequency, in addition to the method of exciting the cantilever in the vicinity of the first-order resonant frequency as described above. In this case, in addition to the oscillating mode in which the tip of the probe oscillates perpendicularly to a long axis direction of the cantilever and in a vertical direction with respect to the sample surface, the cantilever is oscillated in various oscillating modes. A representative oscillating mode includes an oscillating mode in which the probe oscillates torsionally around the long axis of the cantilever.
Further, as an application of the oscillating mode or contact mode atomic force microscope, a physical property such as electrical, magnetical, or optical property, or a mechanical property of the sample is also measured by detecting the physical action at the tip of the probe and the sample surface.
The oscillating mode atomic force microscope using the first-order resonant mode has a merit of causing less damage on the probe and the sample compared to the contact mode atomic force microscope. Further, a signal of the physical property is also detected in synchronization with the oscillation of the cantilever.
On the other hand, the contact mode atomic force microscope has merits of simpler configuration and higher scanning speed compared to the oscillating mode atomic force microscope. Further, the contact mode atomic force microscope may ensure that the cantilever is brought into contact with the sample, and hence is also used for measuring the electrical property of the sample surface.
Further, higher-order oscillating modes are applied to measurements that take advantage of features of the respective oscillating modes. For example, the oscillating mode of the torsional oscillation around the long axis has a feature that the distance between the sample and the probe may be maintained substantially constant during the measurement.
Many measurements by the scanning probe microscope are performed in the atmosphere. However, the cantilever and the sample are placed in a vacuum for measurement when it is desired to eliminate the effect of adsorbed water on the sample surface, vary the temperature of the sample surface, or avoid alteration of the sample surface.
Further, when an organic sample such as a polymer, a cell, a chromosome, a deoxyribonucleic acid (DNA), a protein, or the like, or a biological sample is used, there are cases where the sample and the cantilever are immersed in a solution such as a culture for measurement because the sample is altered when exposed to the atmosphere. The measurement in the solution is applied to in situ observation of a biological sample, an organic polymer sample, and the like, a measurement combined with an electrochemical reaction in the solution, or the like.
Next, referring to FIGS. 5 and 6, a method of controlling the distance between the probe and the sample is described for the oscillating mode atomic force microscope. A current signal generated in the PD 107 in accordance with the oscillation displacement of the cantilever 106a is amplified and converted to a voltage signal by a preamplifier 108. An output from the preamplifier 108, which is an alternate current (AC) signal, is sent to a root mean squared value to direct current (RMS-DC) converter 109 to be converted to a direct current (DC) signal, which corresponds to a root mean squared value.
FIG. 6 is a graph illustrating a relationship between an amplitude amount of the cantilever 106a and the distance between the probe 106b and the sample 101 as the probe 106b and the sample 101 are brought closer to each other from a distance. In FIG. 6, the abscissa represents time in which the probe 106b and the sample 101 are brought closer to each other by the coarse-movement mechanism 103, and the time is converted to the distance between the probe 106b and the sample 101 by being multiplied by a speed of the coarse-movement mechanism 103. The ordinate represents the voltage signal converted by the RMS-DC converter 109, and the voltage signal is converted to the amplitude amount of the cantilever 106a. The change of the signal on the ordinate to the positive side indicates a direction in which the amplitude of the cantilever 106a is reduced. In the scanning probe microscope, a reference signal for measurement is set using the amount of change in amplitude as a parameter in advance in a reference value generating section 111. In the case of bringing the probe 106b and the sample 101 closer to each other, the conventional practice is to bring the probe 106b and the sample 101 closer to each other to a certain extent through manual adjustment by the coarse-movement mechanism 103 while observing by an optical microscope or the like, and thereafter, to further bring the probe 106b and the sample 101 closer to each other by the coarse-movement mechanism 103 until the RMS-DC converted signal reaches the reference signal set in the reference value generating section 111. Note that in bringing the probe 106b and the sample 101 closer to each other, not only the coarse-movement mechanism 103 but also a vertical direction fine-movement mechanism of the three-axis fine-movement mechanism 102 may be used in combination.
As an alternative to the case where the RMS-DC converted voltage is used as the reference signal, there is also a case where a phase difference signal between the voltage applied to the piezoelectric element 105 for exciting the cantilever and the signal detected by the PD 107 may be set as the reference signal in a frequency modulation (FM) demodulator 115.
Further, there is also a mode in which, after a first step of bringing the probe 106b and the sample 101 closer to each other using the phase difference signal as the reference signal, the probe 106b and the sample 101 are retracted once by the vertical direction fine-movement mechanism of the three-axis fine-movement mechanism 102, and the probe 106b and the sample 101 are brought closer to each other up to a measurement area under the same excitation condition by coordinating the vertical direction fine-movement mechanism of the three-axis fine-movement mechanism 102 and the coarse-movement mechanism 103 using the RMS-DC converted voltage signal based on the displacement signal as the reference signal. The phase difference signal starts changing earlier than the RMS-DC converted signal from when the distance between the probe 106b and the sample 101 is still large. Therefore, this mode allows the probe 106b and the sample 101 to approach each other up to the measurement area by bringing the probe 106b and the sample 101 closer to each other at high speed and stopping the probe 106b and the sample 101 reliably at a position at which the probe 106b and the sample 101 do not collide with each other in the first step, and then switching the reference signal to the RMS-DC converted signal.
After bringing the probe 106b and the sample 101 close to each other up to the measurement area, the distance between the probe 106b and the sample 101 is controlled to be constant by feeding back the distance between the probe 106b and the sample 101 so that the amplitude amount of the cantilever 106a is adjusted to the reference signal set in the reference value generating section 111. Specifically, the signal from the RMS-DC converter 109 and the reference signal of the reference value generating section 111 are compared in an error amplifier 110, a signal corresponding to an error is generated in a feedback circuit 112, and a voltage for the height corresponding to the error is applied to the vertical direction fine-movement mechanism of the three-axis fine-movement mechanism 102 through a high voltage amplifier 117. The output from the feedback circuit 112, which is an analog signal, is also converted to a digital signal by an analog/digital (A/D) converter 113, sent to a personal computer for control 114, and imaged as height information. Further, a raster scan signal generated in a scan generator 118 and amplified in the high voltage amplifier 119 is applied to a horizontal direction fine-movement mechanism of the three-axis fine-movement mechanism 102. The raster scan signal and the height information are imaged in the personal computer 114, with the result that the topographic image of the sample 101 may be obtained.
In the scanning probe microscope configured as above, the approach technology for the probe and the sample is very important. Specifically, if the probe and the sample collide with each other when the probe and the sample approach each other, the tip of the probe and the sample surface are broken, to thereby lead to reduced resolution in measuring the topographic image and the damage to the sample. Alternatively, when the speed of the coarse-movement mechanism is lowered to allow the probe and the sample to approach each other more carefully, it takes a long time for the probe and the sample to approach each other, to thereby reduce the measurement efficiency.
Further, in the case of allowing the probe and the sample to approach each other, when the probe and the sample are brought close to each other up to about several tens of μm, the amplitude of the cantilever is gradually reduced by viscous damping due to air that exists in a region between the sample surface and the cantilever. On the other hand, especially when the measurement is performed in the vacuum, the damping due to the air hardly occurs until near the measurement area, and the amplitude hardly changes until the sample and the probe are brought close to each other up to several tens of nm. Therefore, it is difficult to stop the coarse-movement mechanism before causing the collision, and it is more challenging to allow the probe and the sample to approach each other without causing the collision.
In order to reduce the damage to the probe and the sample during the approach as well as to reduce the time it takes for the approach, it is desired to allow the probe and the sample to approach each other at high speed by the coarse-movement mechanism in an area where the probe and the sample do not collide with each other until near the collision, thereafter allow further approach through coordinated operation of the coarse-movement mechanism and the vertical direction fine-movement mechanism, and finally stop the coarse-movement mechanism and operate only the vertical direction fine-movement mechanism when the probe and the sample are brought into contact with each other.
In the conventional method, when the coarse-movement mechanism is operated at high speed, the coarse-movement mechanism may be stopped so late that the probe and the sample may collide with each other. Therefore, in order to reduce the impact in case of the collision or to ensure enough processing time for comparing the RMS-DC converted signal and the reference signal and stopping the coarse-movement mechanism, it has been necessary to lower the speed of the coarse-movement mechanism. As a result, the approach has taken a long time.
In the method including the first step of allowing the probe and the sample to approach each other using the phase difference signal as the reference signal, and a second step of allowing the probe and the sample to further approach each other using the RMS-DC converted signal as the reference signal, the probe and the sample may be allowed to approach each other at high speed without colliding with each other in the first step. However, a special electric circuit such as the FM demodulator is required to perform the measurement using the phase difference signal as the reference signal, and processing for the approach becomes complicated.
Also, it has been impossible to change the excitation condition for the second step because the excitation condition has been the same for the first step and the second step. Especially in the higher-order oscillating modes, it has often been the case that the amplitude or the phase of the cantilever hardly changes until the probe and the sample are brought close to each other up to near the measurement area, which often leads to the collision between the probe and the sample.
Further, in the case of measuring in the contact mode, the probe and the sample often collide with each other to generate a larger impact than in the oscillating mode upon the collision, and hence there has been no choice but to lower the speed of the coarse-movement mechanism.